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DayPaper TUGNOLOGY ’13

LONDONOrganised by The ABR Company Ltd

Conference papers sponsored by

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intrODuCtiOn tO PODsBrief historyElectrical POD drives were developed approximately 20 years ago – the fi rst documented large-sized POD dates back to 1990, when ABB installed a 1.5MW POD on Seili in Finland.1 From 1995 onwards, various larger-sized PODs were installed on cruise vessels such as those belonging to Carnival Cruise Lines. The main manufacturers at that time were ABB, Rolls-Royce and Siemens.

These POD propulsion devices all have the same characteristics – a gondola (cocoon) below the ship wherein an electric motor (E-motor) is fi tted, driving an outboard propeller directly through a shaft. The entire

gondola can be steered in the horizontal plane, thereby delivering fl exible thrust in all directions. The water fl owing around the gondola provides cooling to the E-motor (see Figures 1 and 2).

Although the name POD is commonly-used, the origin of the term is not clear and can either refer to the cocoon-shaped gondola or an abbreviation of Propulsion Outboard Drive.

The delivered ships with PODs have, in general, other electrical power consumers, thereby sharing the generated electrical power both for propulsion and other purposes (eg cruise vessels: hotel accommodation; tankers: pumps).

the v-POD: a new, attractive electric Propulsion

sYnOPsisWith increasing environmental demands, there is a growing need for electric propulsion, and therefore IMC, SARC and Verhaar Omega joined forces to develop a new propulsion outboard drive (POD). This 360-degree rotating electric Verhaar POD (V-POD) propulsion consists of a new arrangement of a medium-speed E-motor and a planetary gear – the main shaft runs from the propeller through the hollow E-motor shaft to the planetary gear on the opposite side. This results in a small, medium-speed E-motor and a compact POD housing. The robust planetary gear enables low propeller revolutions with high propulsion and bollard pull effi ciency. The long prop shaft is supported by sturdy bearings on both sides of the POD housing and offers torsional fl exibility between the propellor and the gears. This paper presents the V-POD development process, prototype testing, the fi rst installation on an inland bunker tanker and the future developments specifi c for the tug market as an alternative to thrusters.

Dr Markus van der Laan (speaker/co-author), IMC Maritime System Development BV, The Netherlands Dick-Jan de Blaeij (speaker/co-author), Verhaar Omega BV, The NetherlandsHerbert Koelman (co-author), SARC BV, The Netherlands

Figure 1: ABB AziPOD CO.2 Figure 2: Close-up internal detail of large direct-drive E-motor.

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Direct-drive PODsAll the PODs presently on the market follow the same layout as described before, and the E-motor drives the propeller directly without a gearbox. The claimed advantages of PODs are:• Lower fuel consumption, eg ABB claims typical

reductions by 5-15 per cent3;• Improved manoeuvrability;• Reduced noise levels;• Reduced emissions.

This last advantage in particular has attracted more attention in recent years. Diesel-electric propulsion enables diesel engines to be operated at a more favourable operational point with reduced NOx and SOx as well as higher efficiency in part-load conditions.

Publications from both ABB and Rolls-Royce focus on the high efficiency of the direct-driven propeller by the E-Motor, but there is another important aspect: propeller efficiency in relation to the revolutions. In many ship applications, the optimal propeller is large and runs at relatively low revolutions. But this requires a high torque on the propeller shaft, and also requires a large and expensive electric motor. Therefore the following applies:

Direct-drive PODs = a compromise between propeller efficiency

and E-motor size/cost.By increasing the propeller revolutions, the torque

lowers and a smaller motor can be applied (size and cost), but the propeller efficiency is also reduced. Although specific details of various ships are not available, many POD-driven ships have significantly smaller diameter propellers than would be selected for reasons of propulsion efficiency (compared to

conventional diesel-shaft-driven propellers). Particularly for ships with sufficient draught, a larger propeller running at lower revolutions in design condition may improve efficiency in a typical range of 5-10 per cent compared to the PODs currently offered4. A brief summary of the pros and cons is shown in Table 1 below.

Although there are clearly certain advantages, the disadvantages have, in general, limited the broader application of PODs and they have only been applied to specific ships with other electrical power consumers.

Geared PODIn many conventional ship designs, a gearbox is mounted between the high(er) speed engine and the low(er) speed propeller. Consequently, a relatively small (and cheaper) diesel engine can drive a large propeller at low revolutions and at high propeller efficiency. In shaft-driven propellers, the suitable gearbox is fitted inside the hull and only the prop shafts are outside. The same also applies to PODs with electric motors (E-motor) and propellers. Here also a smaller (and cheaper) high(er) speed E-motor can drive a large propeller with high efficiency through a gearbox. Although the gear takes some energy (typically 2.5-3 per cent), the propeller efficiency gain is, in general, significantly higher.

Geared PODs = a combination of high propeller efficiency and

small E-motor size/cost.In PODs however, a gearbox requires a detailed

analysis. Several large propulsion manufacturers have investigated various configurations of gearboxes in POD drives, but only limited details have been published. Schottel disclosed the following internal section of its POD in its patent application5 (see Figure 3).

Table 1: Brief summary of the pros and cons of PODs.

Figure 3: Longitudinal section of geared POD.

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The configurations are characterised by the following set-up: propeller – short shaft – planetary gear – E-motor. A planetary gearbox (PGB) has a gear ratio in a range from 1:5-1:7, so the geared E-motor can be five to seven times smaller than the direct-drive E-motor. Since the E-motor price is closely related to the torque, the price of the smaller E-motor is also far cheaper.

If, in addition, the propeller revolutions drop by 20 per cent to improve efficiency and a gear ratio of 1:5 is applied, the E-motor is still four times smaller than a direct-drive motor. This not only results in a smaller and cheaper E-motor, but also in a smaller POD gondola with less water flow resistance and a lower installation size/weight, which clearly illustrates the potential of this concept.

So there is great potential in the development of a geared POD and detailed research has followed in the mechanic/dynamic analysis of the whole drive train.

Without going into full detail, the mechanical design of a geared propulsion system has to meet a number of requirements:

• The propeller blades rotating in a disturbed flow field excitate the shaft with a combination of torque vibrations, axial vibrations and bending moments;

• The shaft in return excitates the gearbox with the ‘remaining’ vibrations and bending moments;

• Inside the gearbox, the gear teeth contact each other with the ‘remaining’ vibrations and bending moments;

• Gear teeth are particularly vulnerable to vibrations and bending moments. In particular, non-parallel loading and temporary loss of contact between the teeth face should be prevented.

In order to reduce vibrations and bending moments from the propeller to the gear, the propulsion train should include as much mechanical flexibility as possible. However, this conflicts with the concept:

• A stiff and short prop shaft is needed to meet the vibrations and bending moments;

• The short shaft introduces high radial bearing forces;

• The shaft forms a rigid part with the planetary gear drive and transfers the vibrations and bending moments;

• These loads are directly transferred to the gear teeth, limiting the operational life.

This short, qualitative analysis demonstrates clearly that the mechanical design conflicts with the given constraints. Although various companies have investigated the Geared POD concept in detail, this has not resulted in POD designs being applied to shipping, presumably because of the mechanical restrictions on the design.

A detailed internal section of a Geared POD is shown in Figure 4. The mechanical design is based on a stiff and short prop shaft forming an integral part with the planetary gearbox and short bearing distance.

DevelOPment Of the new GeareD POD: the v-PODWith increasing environmental demands, there is a growing need for diesel-electric propulsion and therefore IMC, SARC and Verhaar Omega joined forces to develop a new POD. In summer of 2010, the analysis and evaluation of the Geared POD started with a simple objective:

Create mechanical flexibility between propeller and the planetary gearbox.

new conceptual designUsing the IMC methodology of systematic innovation, two separate concepts were developed based on a long shaft running from the propeller end of the gondola to the opposite side: 1. A number of slender E-motors positioned around

the central shaft all driving a central gear at the opposite end (see Figure 5 on next page).

2. A single E-motor with a hollow shaft around the

Figure 4: Longitudinal section of Geared POD.

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prop shaft driving a PGB at the opposite end using a hollow sun shaft (see Figure 6).

Figure 6: Hollow E-motor and planetary gearbox.

Both options were developed into a preliminary mechanical design to validate the feasibility. After detailed consultation with E-motor and PGB manufacturers, it became clear that option 2 was more realistic, more compact in size and more cost-attractive. Therefore, this option was selected for the development of the V-POD for Verhaar Omega.

mechanical and electrical designWith the basic concept fixed, the engineering began. Many rather standard components were selected, but combined in a completely different setting with hollow shafts. Various components are described here in drive order:• motor/hollow shaft

Initially, E-motor manufacturers were conservative and only allowed for small diameter openings through the rotor, but gradually, more detailed analysis disclosed that in conventional AC motors, a significant opening can be made in the rotor shaft without losses in magnetic field. By increasing the pole number from 4 to 6 or even 8, the opening can reach the required size. Permanent Magnet (PM) motors were also investigated, and although a higher power density is achieved than for AC

motors, the higher price was not attractive. Also, the marginal efficiency advantage compared to AC motors was not considered sufficient in relation to the price level (there are still questions on long-term PM efficiency).

• Planetary gear A planetary gear balances the load from the sun-shaft to all three planetary gear wheels and, in order to provide flexibility, a spline connection is used between the E-motor rotor shaft and the sun-shaft. As in case of the E-motor, the planetary gear manufacturers were conservative and allowed at first for only small diameter openings through the sun shaft. Finite element analysis demonstrated, however, that with a typical gear ratio near 1:4, the opening size was possible (see Figure 7).

Figure 7: Finite element specially-designed sun shaft.

• Propeller shaft connection to planetary gear by a spline The plan gearbox drives the prop shaft without taking axial loads and bending moment. Therefore, a spline is also used in this connection, thereby ensuring proper loading on the gear teeth.

Figure 5: Four E-motors around one central shaft.

Figure 8: Finite element modelling of prop shaft tapering/flexibility.

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• Slenderflexibleshaftuseoffiniteelementmodelling/shaping The load/bending moments of the prop shaft result in an optimal shape – a thick shaft towards the propeller, the maximum thickness at the thrust bearing and a long slender/tapered shaft to the opposite side of the gondola. The shaft analysis has been carefully investigated and shaped with finite elements in order to obtain proper flexibility with significant safety margins (see Figure 8 opposite).

• triple sealing Functioning of the POD requires good fluid seals and monitoring systems. In the design of the V-POD, a safe approach is used by applying a triple-seal barrier: a double-seal barrier from outward water to the lubricating system and an additional seal barrier from the lubricating system to E-motor. All seal barriers are equipped with a detection system and alarm. Furthermore, the E-motor windings have been selected to withstand the lubricating oil in the unlikely event of leakage into the E-motor compartment. And finally, the E-motor compartment is equipped with a sensor and constant pumping system to directly alert any possible leakage.

• Lubricating/coolingsystem The entire internal POD is part of a continuous

Example 500kW unit Geared POD V-POD Propeller Same excitation Same excitation Propeller shaft Short: L = 550-600 mm

Stiff: D = 140 mm Long: L = 1500 mm Flexible: D = 140 95 mm

Shaft connection Rigid shaft PG drive: Direct transfer of axial vibrations and bending moments

Spline connection: No transfer of axial vibrations and bending moments

Teeth in gear box Remaining vibrations high causing significant teeth wear

Remaining vibrations low causing minimal teeth wear

Sturdy tug applications Not suitable Suitable design

Table 2: Comparison of Geared POD and new V-POD.

Figure 9: Longitudinal section of the V-POD prototype showing the main components.

flowing lubrication system, thereby offering internal cooling, but more importantly, a constant monitoring of all components in the POD housing. The flow is forced by a small pump and adjusted according to the operating speed. Monitoring includes temperature, flow speed, water detection and particle detection.

• electric power supply The electric motor inside the V-POD requires power from the ship, and therefore a rotary electrical power system is needed. Two alternatives are offered here: either a sling-ring unit for unlimited rotations, or a power chain unit for limited rotations (eg 270-degrees).

• In Table 2 (below), the mechanical differences between

the Geared POD and the V-POD are summarised.

ThefinalproductAfter many hours of engineering and consultation with various contributing companies, the final dimensions and components were selected. The result: the first V-POD prototype. The complete longitudinal section of the V-POD is shown in Figure 9, below, and its main characteristic components are shown in Figures 10-13 on the following pages.

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Figure 10: Hollow E-motor rotor shaft for prop shaft.

Figure 11: Hollow sun-shaft and spline to rotor shaft.

Figure 12: Planetary gearbox with three gear wheels.

Figure 13: Main thrust bearing (half bearing removed).

testing: ‘the proof of the pudding is in the eating!’In order to test the new concept, a prototype with 330kW was designed, with sufficient size to demonstrate its function in real life and to convince the market. The process continued with prototype design, component selection and modifications, gondola-casting with slender hydrodynamic shape, a special E-motor with hollow rotor and, finally, a modified planetary gearbox with special hollow sun shaft. First trials were performed at the Verhaar factory in Oegstgeest on a fixed frame in the river (see Figure 14), and next, tests were performed mounted under an inland tanker for real-life endurance testing (see Figure 15 opposite).

Figure 14: River trials on fixed frame at Verhaar.

Rigorous testing has disclosed some minor issues in the design, but the general concept of the long flexible prop shaft through the hollow E-motor shaft, combined with the plan gearbox was sound and sturdy. The lessons learnt from the prototype resulted in some minor modifications and were included in the final design for the market.

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Figure 15: Real-life testing on inland tanker.

v-PODs delivered and operatingIn 2011, contacts started with owners Mandemaker/Molegraaf and shipbroker Rensen-Driesen on the design of a new series of diesel-electric inland tankers. Both the good reputation of Verhaar Omega and the sound mechanical design of the V-POD convinced the parties to proceed and build a whole series of tankers, all powered by V-PODs.

The first one, the inland tanker Zeus (LxB = 60x7m; 330kW power; 600-ton cargo) was built in 2012, successfully completed its trials in October 2012 and has been in service since then. Although there have been some minor issues, primarily related to the electrical integration, all these have been solved and the owners, Mandemaker/Molengraaf, are very satisfied. Recently, its sister vessel, Themis, successfully completed trials and joined service in January 2013. Various pictures of Zeus are shown in Figures 16-19.

Figure 16: The V-POD mounted under Zeus.

Figure 17: The internal slewing gear and slip ring.

Figure 18: Full power thrust of the V-POD.

Figure 19: Zeus powered by the V-POD.

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The next two larger-sized inland tankers, Metis and Kratis, (LxB = 80x9.5m; 2 x 330kW power; 1,200-ton cargo) each have a double V-POD arrangement and are under construction.

v-POD vs thruster COmParisOnSince the benchmark propulsion in the tug industry is an azimuth thruster, a brief technical comparison between the thruster and the V-POD is given below:• Gear angle: Thrusters require special and precisely-

manufactured cyclo-palloid gearing teeth to transfer the high propeller torque and all related vibrations. When wear occurs in bearings, the cyclo-palloid gears are vulnerable, because the exact interacting teeth are essential. The V-POD uses straight teeth in a planetary gear system, and is less vulnerable because of large tolerances in axial deviations and bending moments due to the spline connection.

• Numberofgearsets: Thrusters have, in general, two gear sets: one lower gear inside the gondola and a second gear inside the hull to connect to the diesel engine shafts. The V-POD has only one single gear set inside the gondola.

• acceptable torque: The torque of thrusters is largely related to the production price, and therefore often higher prop revolutions are selected with lower torque. The V-POD has a large torque margin for two reasons: a) the load is distributed over the number of planet gear wheels and b) the hollow shaft design results in relatively large gear wheels. Prop revolutions can therefore be lowered down to the level needed for propulsion optimisation.

• manufacturing price: Cyclo-palloid gear prices increase significantly with increasing torque, whilst plan gear prices increase slowly with increasing torque. With increasing torque, the price advantage of the planetary gear systems increases simultaneously.

• mechanical design: Thrusters have a short and stiff connection between the prop and the gears, whereby vibrations are directly transferred into the gear teeth, with, in the long term, increased wear and reduction in operation life. The V-POD has a relatively long and flexible shaft, whereby vibrations are lowered before entering the gears, with less wear and longer life time.

• mass and inertia: The steering part of a thruster involves the gondola, shaft and gears and outside, the prop and nozzle. The total mass and inertia are relatively low. In the case of the V-POD, the mass and inertia are higher due to the E-motor and therefore the required slewing gear bearings and steering module are one step larger than for thrusters. However, the mass and inertia of the V-POD is still significantly smaller than that of direct drive PODs.

Considering the above, the V-POD offers a serious competitor to thrusters.

aPPliCatOn Of v-POD fOr tuGsPrevious examples have shown the application of the V-POD on inland ships; here, as an example for the

tug industry, a typical 24m ASD harbour tug is drafted with two V-PODs (see Figure 20 opposite) (Series II; propeller diameter 2,200mm; power range 0.75-1MW; typical bollard pull 17-20-tonnes).

typical characteristics:• Large propeller diameter with good BP;• Slender gondola shape of the POD housing

(approximately 30 per cent of propeller diameter);• Inside hull:

– Typically 3-4 diesel generator sets and controllers; – In thruster room only steering gear and rotary electric supply; – No long shaft-lines between engine room and thruster room.

• Free economic sailing with two V-PODs can already be performed with one gen-set, reducing fuel consumption, lowering emissions and reducing running hours/maintenance.

• The general advantage of diesel-electric and hybrid propulsion for (harbour) tug use with large variations in power load has already been discussed extensively in various papers at ITS/Tugnology conferences6. The aim here is to demonstrate that the V-POD offers a practical component for electric propulsion.

COnClusiOns anD reCOmmenDatiOns• Direct-drive PODs have been applied in shipping

for over 20 years, but are primarily limited to specialised vessels with shared power consumers;

• One of the main reasons is the high price of the direct drive POD;

• The second reason is the compromise between propeller efficiency and motor size, leading, in general, to lower propeller efficiency;

• A ‘geared’ POD offers interesting perspectives to reduce the size and cost of the E-motor combined with improved propeller efficiency;

• The short and stiff ‘in-line’ shaft geared concept (investigated by various manufacturers) cannot reach satisfactory mechanical solutions;

• The new V-POD concept of a long and flexible prop shaft through a hollow E-motor shaft can reach satisfactory mechanical solutions;

• The planetary gearbox offers a robust and long-lasting solution;

• The new POD propulsion combines a lower price with higher efficiency;

• For tugs with low propeller revolutions for bollard pull, the V-POD offers a particularly attractive solution;

• Diesel-electric propulsion combined with V-PODs is now within ‘reach’ for a wider range of ships;

• Other fuel generators, such as LNG/CNG, or hybrid power, can be combined with the electric V-POD, offering interesting perspectives for the future;

• Present power range of V-POD up to 1MW, but gradually increasing power range in the next few years.

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Figure 20: Sketch of V-POD under typical 24m ASD harbour tug.

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aCKnOwleDGementsThe authors wish to express their sincere thanks and appreciation to the following contributors to the V-POD project: Green Shipping CV (Mandemaker/Molegraaf/Slingeland), Rensen-Driessen Shipbuilding BV, Breko Shipbuilding and Lloyd’s Register.

1 First installed 1.5MW ABB POD on Seili in Finland.*2 Picture showing internal ABB AziPOD XO 2010.*3 Description of claimed advantages for ABB AziPOD systems.*4 Based on ABB AziPod technical brochures, AziPOD CO brochure_2010.pdf (removed from original ABB site).*5 Schottel patent: Propelling drive apparatus (Patent EP 1 972 545 A1).* See also recent patent of Schottel: Rudder propeller with an underwater mechanism comprising a planetary gearing (seems to indicate that a planetary gearbox offers interesting advantages over cyclo-palloid bevel gears).*6 Various papers on diesel-electric power at ITS and Tugnology conferences:ITS 2008 Singapore The Advantages of Diesel Electric Propulsion Systems for Offshore Vessels, Vince Rodomista, Caterpillar Marine Power Systems, USA.Tugnology 2009 Amsterdam The Hybrid Tug Reality – the Business Case for Green Technology in the Tugboat Industry, Jason Aspin, Aspin Kemp & Associates, Canada, and Susan Hayman, Foss Maritime Company, USA.

referenCesLinks to the reference documents marked * can be found at http://www.tugandosv.com/tugnology2013-references

ITS 2010 Vancouver The Foss Hybrid Tug: The Journey into the Future, Susan Hayman and Elizabeth Reynolds Boyd, Foss Maritime Company, USA.Update of the E-3 Tug Development, Roderique Van Erp, Smit Engineering, The Netherlands.Project to Build a Hydrogen-Powered Hybrid Electric Tug, Chris Mulder & Mark Mulligan, Capillano Maritime Design, Canada.Lithium Power: Energy System for the Next Generation of Vessels, Brent Perry, CEO, Corvus Energy Ltd, Canada.ITS 2012 Barcelona Conversion of Kotug’s RT Adriaan to become Europe’s First Hybrid Tugboat, Paul Jamer, Aspin Kemp & Associates, Canada.Hybrid Systems – Aiming for a Better Compromise, Arnstein Eknes and Bjørn Johan Vartdal, Det Norske Veritas, Norway.

General references to PODs (not numbered in text):7. ABB AziPODs.*8. Rolls-Royce Marine Propulsion website.*More information on the V-POD: www.verhaar.com